This invention pertains to the general field of lubrication, and specifically comprises a new and improved plunger for use as a piston in grease guns and like applications.
The art of grease guns is one in which much inventive activity has taken place. In Helgerud U.S. Pat. No. 3,780,830, assigned to the assignee of the present application, the general nature of a grease gun is clearly disclosed, and it is taught that when the gun is filled with grease a spring acts on a plunger at one end of the gun to urge the mass of grease toward the other end, where a simple reciprocating pump is located for manual operation to first clear a subordinate chamber for filling with grease from the gun and then force a piston into the chamber for expelling the grease under high pressure past a suitable check valve to the fitting being lubricated. The gun may be filled either by use of prepared cylindrical grease cartridges or from a bulk supply of grease, and accordingly the plunger must be capable of tightly engaging either with the inner wall of the gun itself, or with the slightly smaller inner wall of a grease cartridge, being neither too tight to function properly in the cartridge nor so loose as to permit bypassing of bulk grease in the cylinder. Known plungers have not proved satisfactory for this use.